In many areas, such as in the application of movable parts in the automated fabrication or processing of work pieces, the precise determination of spatial positions is of considerable relevance in order to, e.g., be able to fabricate or process components having low tolerance in an efficient manner. For example, the material processing of work pieces by a working laser beam requires a precise positioning of the working laser beam or working beam on a work pieces to be processed. The laser beam of a working laser is directed, e.g., by means of a galvanometer scanner, which comprises a scan mirror or working beam deflection mirror which is attached to a rotatably disposed shaft of a galvanometer drive, via reflection at the working beam deflection mirror to a desired spatial position, wherein the shaft and thus also the working beam deflection mirror can be set in rotation via a motor such that the optical path of the working laser beam can be spatially varied. By connecting two such galvanometer scanners in series, the laser light can be deflected in two dimensions. An exact control of the optical path requires a precise determining of the respective rotation angle positions of the shaft (and thus of the working beam deflection mirror).
For measurement of the angle position, e.g. in a galvanometer scanner, capacitive position detectors are often used, wherein, e.g., the rotatably disposed shaft is coupled to an electrode of a variable capacitor or to a dielectric which is disposed between the electrodes of a variable capacitor, and wherein the present rotation angle results from the measurement of the capacity of the variable capacitor. Such capacitive position detectors are often complex in design, have a high moment of inertia and show a temperature-dependent behavior.
Optical position detectors, in which a light blocker which is rotatably disposed by means of a shaft is illuminated by a light source and shadows more or less light depending on the angular position, are also often used. The present rotation angle then results from, e.g., the amount of light which passes the light blocker and which is measured by a corresponding detector. Such optical position detectors often have a distinctive temperature dependency and temporal drift and can show a non-linear output characteristic at relatively small resolving power.
Alternatively, position detectors with movable optical gratings are used, wherein, e.g., a circular scale disk having a scale pattern or line pattern which is provided in the vicinity of the outer circumference of the disk is attached to the rotatably disposed shaft, and wherein the rotation angle position of the shaft can be determined by optical scanning of the scale pattern. Normally, such position detectors have good linearity as well as small temporal drift and deliver a high angle resolution. However, the moment of inertia of the assembly which is to be moved by the motor of the shaft is substantially increased by the scale disk, the more so as the achievable angle resolution is the higher, the larger the diameter and thus also the moment of inertia (which strongly increases with the diameter) of the scale disk is. This increase of the moment of inertia increases the inertia of the positioning system and thus complicates a fast repositioning of the shaft to another rotation angle position. The dynamics of such a system is reduced amongst others by the lower eigenfrequencies, which are caused by the high moment of inertia, and is thus deteriorated.